Bind9 Returning SERVFAIL for custom Zones

I'm trying to setup a DNS-Server for my Domains using Bind. The Server behaves correctly for noncustom Domains like google.com or github.com, but for my domains it always returns "SERVFAIL". I've already checked the zone file using named-checkzone which returns "OK".

My Zone config (defined in named.conf.local):

zone "michlfranken" {
  type master;
  file "/var/cache/bind/db.isiko404.dev";
};

My Options File:

options {
        directory "/var/cache/bind";

        // If there is a firewall between you and nameservers you want
        // to talk to, you may need to fix the firewall to allow multiple
        // ports to talk.  See http://www.kb.cert.org/vuls/id/800113

        // If your ISP provided one or more IP addresses for stable
        // nameservers, you probably want to use them as forwarders.
        // Uncomment the following block, and insert the addresses replacing
        // the all-0's placeholder.

        forwarders {
                80.241.218.68;
                46.182.19.48;
        };

        //========================================================================
        // If BIND logs error messages about the root key being expired,
        // you will need to update your keys.  See https://www.isc.org/bind-keys
        //========================================================================
        dnssec-validation yes;

        listen-on-v6 { any; };
        listen-on { any; };
};